NEW ORLEANS — BP could be looking at nearly $18 billion of additional fines over the nation’s worst offshore oil spill after a federal judge Thursday ruled that the company acted with “gross negligence” in the 2010 Gulf of Mexico disaster.

U.S. District Judge Carl Barbier concluded that the London-based oil giant showed a “conscious disregard of known risks” during the drilling operation and bears most of the responsibility for the blowout that killed 11 rig workers and spewed millions of gallons of oil for three months.

In the next stage of the case, set to begin in January, the judge will decide precisely how much BP must pay.

Under the federal Clean Water Act, a polluter can be forced to pay a maximum of $1,100 of civil fines per barrel of spilled oil, or up to $4,300 per barrel if the company is found grossly negligent. Barbier’s finding exposes BP to the much higher amount.

Even as the oil giant vowed to appeal, BP stock fell $2.82, or nearly 6 percent, to $44.89, reducing the company’s market value by almost $9 billion.

“Everybody talks about how big they are, but it’s staggering,” David Uhlmann, a University of Michigan law professor and former chief of the Justice Department’s environmental crimes section, said of the price tag for the spill.

BP had agreed to pay a record $4 billion of criminal fines and penalties over the Deepwater Horizon disaster, plus more than $27 billion of cleanup costs and compensation to people and businesses harmed by the spill.

BP made $24 billion of profit last year but could be forced again to sell some assets to cover the additional fines, analysts said.

Attorney General Eric Holder said Barbier’s ruling “will ensure that the company is held fully accountable for its recklessness” and will “serve as a strong deterrent to anyone tempted to sacrifice safety and the environment in the pursuit of profit.”

Barbier held a non-jury trial last year to identify the blowout’s causes and apportion blame for the disaster.